The authors present an unusual case of a man with a nontraumatic tear of the flexor carpi radialis muscle after a regular swimming exercise, without known precipitation factors. The muscle tear was diagnosed by ultrasonography, which showed a cystic mass with increased peripheral vascularity, and was confirmed by magnetic resonance imaging. After conservative treatment, follow–up ultrasonography showed resolution of the hematoma. The patient was able to continue swimming without pain or limitation of function.